1. Field of the Invention
The present invention relates to AC-DC transformers where the AC power supply is of the type commonly available in homes, offices and the like, and the DC power supply is compatible with electronic devices such as radios, cellular telephones and the like. In particular, the present invention relates to transformers having a plug for plugging into a typical AC power source outlet, wherein the plug is foldable so that the transformer can be compactly stored when not in use.
2. Description of the Prior Art
Power transformers are often used with portable devices, and have been developed to be easily transported along with portable devices. Accordingly, such transformers are typically small, light, convenient to manufacture and safe to use.
A conventional foldable transformer is shown in FIG. 7. The transformer includes a housing 100 having a storage cavity 102 at a top end thereof, a plug assembly 103, and wires 108. The plug assembly 103 is moveable between a folded position for storage and an unfolded position for use. The plug assembly 103 includes a rotatable rod 106, and a pair of pins 104 for connecting with an external power source. The pins 104 are made of conductive material, and are insert-molded in the rod 106. The wires 108 are respectively soldered at each end of the pins 104 in the rod 106. Thus the pins 104 can both rotate together with the rod 106 between the folded position and the unfolded position, and are easily connected with the wires 108 to transmit power from an external source to a power transforming circuit (not shown) within the transformer itself. Because the pins 104 and the wires 108 are soldered together, rotation of the pins 104 generates stress to the solder joints between the wires 108 and the pins 104. After repeated use, the solder joints are liable to become fatigued. Electrical connection between the wires 108 and the pins 104 may be lost, whereupon power transmission from the pins 104 through the wires 108 to the power transforming circuit is lost. Thus the transformer is prone to be unreliable after repeated use.
Another conventional transformer is shown in FIG. 8. The transformer includes a housing 200 having a storage cavity 202 at a top portion thereof, and a plug assembly 203. The plug assembly 203 further comprises a pair of rotary pins 204, a pair of fixed pins 206, a rotatable transverse rod 208, an engaging block 210, and a spring 212. Each rotary pin 204 is pivotally connected to a corresponding fixed pin 206 via a pivot located at an end of the rod 208. The rotary pins 204 are pivotally foldable while maintaining constant electric connection with the fixed pins 206. The rod 208 also has a groove 209, and the engaging block 210 has an engaging ridge 211 for engaging in the groove 209. A blind hole 213 is defined in a side of the engaging block 210 opposite to the engaging ridge 211, for receiving one end of the spring 212. The storage cavity 202 further has a pair of pin holes 215 for receiving the fixed pins 206, and an opening 217 for receiving the other end of the spring 212 to thereby fix the plug assembly 203 to the housing 200.
In assembly, each fixed pin 206 is inserted into a corresponding pin hole 215. When the rotary pins 204 and rod 208 are rotated to an unfolded position, the spring 212 exerts force such that the ridge 211 firmly engages in the corresponding groove 209. The pins 204 are thus securely held in position, allowing the plug to be conveniently inserted into an AC power source outlet.
The above-mentioned foldable transformer does not have wires directly soldered to the rotary pins 204. Thus the problem of the transformer of FIG. 7 is resolved. However, the above-mentioned foldable transformer has at least the following drawbacks:
First, it has many components and a complicated structure, making it unduly difficult to manufacture.
Second, the spring 212 is not a reliable component. In particular, the force exerted by various springs 212 is not always constant. Thus the quality of the plug assembly 203 is not always uniform. Secondly, after repeated use of the plug assembly 203, the resiliency of the spring 212 may be reduced to an unworkable level.
Accordingly, a foldable transformer having an improved plug assembly is desired.
Accordingly, an object of the present invention is to provide a plug assembly for a foldable transformer that has a reduced number of components, is easily assembled and manufactured, and is reliable.
To achieve the above object, a foldable transformer in accordance with a preferred embodiment of the present invention comprises an enclosure, a plug assembly and a locating assembly. The enclosure has a storage cavity at a top end thereof. A window is defined in the enclosure in communication with a bottom portion of the storage cavity, for receiving the plug assembly therethrough. A pair of tabs extends inwardly from the enclosure above the window, and a locating block extends inwardly from each tab. The plug assembly includes a pair of inserting pins, a rotatable rod having an arched profile, a pair of metal ears, and a pair of spring pins. Each inserting pin is made of conductive material, is insert-molded in the rod, and electrically connects with a corresponding ear. A protrusion is formed on a central portion of an apex of an arched surface of the rod. A pair of parallel projections extends outwardly from an arcuate portion of the arched surface of the rod proximate the protrusion. Each spring pin is made by folding a metal strip back over itself. Each spring pin comprises a first portion and a second portion connecting with each other via a neck. A dimple is formed in a free end of the second portion, to increase a contact area between the spring pin the corresponding metal ear. The locating assembly includes a pair of slots for receiving the spring pins, and a cam for resiliently engaging with the plug assembly so that the plug assembly can be disengagably fixed in a folded position or an unfolded position. The locating assembly also includes a pair of locating holes for locating the locating blocks.